Automated Migration for Breaking Changes to the Type System

Automated Migration for Breaking Changes to the Type System ## Search Terms codemod, migration, upgrade ## Suggestion semi-automated migration when there are breaking changes in the type system One way of implementing this would be via codemods (could use the TS Compiler API) that do trivial update tasks as described in https://github.com/microsoft/TypeScript/issues/33272 ## Use Cases - Would make it easier for users to upgrade to the latest TypeScript - Would make it easier for the TS team to upgrade TypeScript - Might enable TS to evolve faster, as the pain from these "breaking" changes will be smaller The changes described in https://github.com/microsoft/TypeScript/issues/33272, for example, were mostly trivial, and ideally would not need much human intervention. Type-only changes are especially amenable to this kind of treatment, because no one's app will misbehave at runtime if there is a bug in the transformation. I say 'semi-automated' because in cases where the tool can't figure out the problem, the tool could yield to human judgment. Tooling for this doesn't have to be perfect, but a small amount of automation could go a long way. ## Examples example.ts ```ts const myPromise: Promise<{}> = dontCarePromise(); ``` tsc --upgrade --from=3.5 --to=3.6 example.ts ```ts const myPromise: Promise<unknown> = dontCarePromise(); ``` The example is from https://github.com/microsoft/TypeScript/issues/33272. In a case where the conversion *would* lead to a new type error, I'd expect the tool to bail out, explain the situation, and let the human decide what to do. ## Checklist My suggestion meets these guidelines: * [x] This wouldn't be a breaking change in existing TypeScript/JavaScript code * [x] This wouldn't change the runtime behavior of existing JavaScript code * [x] This could be implemented without emitting different JS based on the types of the expressions * [x] This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, …